...on the recent Mozart disc with Colin Lawson, clarinet:

Mozart composed his Clarinet Quintet and Clarinet Concerto, as well as the Trio for clarinet, viola and piano K498, for the clarinettist Anton Stadler, who gave the first performances of the quintet and the concerto using the basset clarinet, which extended the instrument's lowest register by four semitones. Colin Lawson uses a modern copy of such an instrument for his performance of the quintet with the period strings of the Revolutionary Drawing Room; there's a wonderful smoky smoothness to his tone that's distinctly different from the sound of the period clarinet he uses in one of the Mozart fragments that complete the disc, the Andante Rondo for B flat clarinet ad string quartet. The others include the first ever recording of a completion of an A major Rondo that was originally intended for the quintet but instead was recycled as an aria for Così fan Tutte, and a movement for clarinet, basset horn and strings, which might well have been intended for Stadler and his younger brother Johann.